The invention relates to the field of transmitters and receivers for radio frequency communication systems, and particularly relates to combined circuits for transmitting and receiving radio frequency (RF) signals.
As wireless communication systems have become increasingly popular, a demand has developed for less expensive yet spectrally clean RF transmitters. High quality RF transmitters typically include relatively expensive components. For example, certain bandpass filters, such as surface acoustic wave (SAW) filters provide excellent performance yet are relatively expensive. Many applications further require transmitters that exhibit low power consumption. It is also desirable that transmitters be suitable for use with any of a plurality of standards for modulation, e.g., global system for mobile communication (GSM) or digital cellular system (DCS).
It is conventionally known that transmitter circuits should be designed to reduce the possibility of spurious signals (e.g., harmonics as well as foreign signals) being introduced into the system. In certain situations, the origin of some spurious signals may be extremely difficult to predict, and may be nearly impossible to simulate. To address this problem, it is generally believed that conventional transmitter circuits should be designed to be flexible so that the frequency plan may be adjusted to remove any noise from the band of interest.
For example, in certain situations, a circuit may be most easily corrected by employing two separate oscillators that facilitate adjustment for reducing noise since either may be adjusted independent of the other. Moreover, the frequencies may be chosen so as to not be harmonically related, which minimizes the chance of harmonic spurious signals being produced by the oscillators. Unfortunately, however, some oscillators are rather expensive. For example, certain oscillator circuits that are formed of synthesizers produce very stable output signals, but are relatively expensive. It is also desirable that the use of relatively expensive filters be avoided.
There is a need, therefore, for inexpensive yet efficient RF transmitters. There is further a need for a translation loop modulator that is spectrally efficient yet economical to produce.
RF receivers generally either convert an input RF signal to an intermediate frequency (e.g., a superheterodyne receiver), or directly mix an input signal to a direct current (DC) signal (e.g., a direct conversion receiver). A direct conversion receiver mixes directly to a DC signal, and is sometimes referred to as a zero IF receiver because the intermediate frequency is zero Hertz (DC). The modulation information only is represented in the down conversion, and there is no carrier information that is typically associated with an intermediate frequency. In a direct conversion receiver the local oscillator signal is operating at the same frequency as the input RF signal. U.S. Pat. Nos. 5,438,692 and 5,548,068 disclose conventional direct conversion receivers.
In direct conversion, the modulation information is preserved through quadrature down conversion, which involves mixing the incoming line or carrier with a local oscillator signal along two different paths. The local oscillator signal along one path may be at zero phase (0xc2x0) with respect to the input RF signal, and may be phase shifted to 90xc2x0 along the other path. Alternatively, one path may be at xe2x88x9245xc2x0 while the other is at +45xc2x0 with respect to the input signal. See for example, U.S. Pat. No. 5,303,417. In any event, the circuit paths are typically mutually 90xc2x0 different in phase, and one path is referred to as the I channel while the other is referred to as the Q channel. The quadrature down conversion method preserves the necessary phase information within the input signal.
Interference may occur if the local oscillator signal radiates to the input RF signal. Because the frequencies of these signals are the same, the local oscillator signal cannot be frequency filtered from the incoming signal. The incoming signal would, in effect, be blocked. U.S. Pat. Nos. 4,811,425 and 5,428,837 are directed to reducing the effects of leakage of local oscillator signals to RF input signals in zero IF receivers.
Moreover, interference may occur if the RF input signal radiates to the voltage controlled oscillator (VCO). Since VCOs are typically very sensitive, any signal that is close in frequency to the frequency of the VCO may interact with it, even if the signal comprises only a small amount of energy. This is because the VCO will selectively amplify signals that are close in frequency to the VCO signal, causing spurious responses.
One way of overcoming this problem is to employ a VCO that operates at a frequency different than the input RF signal. The frequency of the VCO signal is then modified to produce a local oscillator signal at the same frequency as the input RF signal. For example, the signal from one VCO (at frequency F1) may be combined with the signal from another VCO (at frequency F2) by a mixer. The combined signal may then be filtered by a bandpass filter to produce a local oscillator signal. The product, however, of the F1 and F2 signals, will include spurious signals that must be filtered out to produce a spectrally clean local oscillator signal. For example, the product of two sine functions sin(xcex1)xc3x97sin(xcex2) equals xc2xd cos(xcex1xe2x88x92xcex2)xe2x88x92xc2xd cos(xcex1+xcex2). Two frequencies would be produced at the mixer (F1+F2) and (F1xe2x88x92F2), and one would have to be filtered out. It is typically necessary to do this type of filtering off IC, which further invites interference or leakage of the local oscillator signal to the input RF signal.
In other conventional local oscillator circuits, one VCO only might be employed and the output of the VCO would be input to a frequency doubler, and then to a filter. The frequency of the VCO (F1) could be one half the frequency of the RF input signal, and the frequency of the local oscillator would then be 2F1. In further conventional local oscillator circuits, the frequency of the VCO (F1) could be twice the frequency of the RF input signal, and the frequency of the local oscillator signal may be equal to xc2xdF1. This could also be achieved with one VCO (F1), whose output could be input to a divide-by-two circuit to produce the local oscillator signal. In each such circuit however, the local oscillator signal may still radiate to the RF input signal, and the VCO may be sensitive to harmonic frequencies of the RF input signal.
Such conventional techniques do not fully alleviate the interference problems. It is a further object of the present invention to provide a direct conversion receiver or transmitter that has reduced leakage or interference between the radio frequency input signal and the local oscillator.
It is also an object of the invention to provide an improved RF transmitter and receiver in a single system that achieves the above objectives.
The invention provides transceiver circuit for use in radio frequency communication systems. The circuit includes a transmitter circuit, a receiver circuit and a local oscillator circuit. The local oscillator circuit includes at least one oscillator input signal having a frequency that is a non-integer multiple of the transmission frequency of the radio frequency communication system. The oscillator input signal is used to produce a transmitter local oscillator signal and a receiver local oscillator signal. In an embodiment, the frequency of two bands of a dual band radio frequency input signal (FRF) are provided by the modulus of the product of the frequency of the local oscillator (FLO) multiplied by the sum of a non-integer value (1/x) plus or minus one respectively.